The invention relates to an apparatus and method for downhole injection of a radioactive tracer. At least two boreholes are used. Progress of the radioactive tracer forced through fractured structure from one borehole to another is followed by at least one downhole detector.
A prior art practice of determining the nature of interborehole fractured structure utilizes radioactive .sup.131 I. After it is poured down a borehole, water is pumped into the hole to force the iodine tracer through the fractured structure. Several problems and disadvantages exist when iodine is used. First of all it is very expensive. Secondly, the tool used remains radioactively hot for at least three months and must be stored and not used. Extreme care must be exercised in withdrawing and storing the tool because it is contaminated. Thirdly, workmen in the field tend to be cavalier with the iodine and it slops on the ground, on their shoes and clothing, and the like. Therefore not only is the iodine itself expensive but it is costly to use and there are many hazards associated with its use. A preferred embodiment of the invention uses a radioactive isotope of .sup.82 Br enclosed within a capsule in granular form. The half-life of the bromine is 11/2 days compared to the 8 day half-life for .sup.131 I, meaning .sup.82 Br is safer from this standpoint. Too, the nonselective biological response of the .sup.82 Br provides greater safety in addition to its added usefulness ,ecause of its short half-life. Workmen need not handle radioactive material itself in practicing the invention. There is thus no occasion for it to be slopped on the ground or onto a workman's shoes or clothing. In addition, because the tool head is repeatedly flushed with water, upon its return to the surface it can be immediately handled by workmen with complete safety. The radioactivity of the tool head can be monitored by a detector disposed in its proximity, to ensure that it is essentially free of radiative material before it is returned to the surface. Because of these features, workmen can safely handle a tool with bare hands immediately upon its retrieval from downhole; the tool can be immediately reused and need not be put on the shelf for three months as when .sup.131 I is used.